This invention relates to the fields of medical devices, medical diagnostics, and cell counting.
Microfluidic systems have shown unique promise for studying cell function, cell and tissue engineering, disease diagnosis, blood sample preparation, and drug discovery. Very recently, the use of microfluidics to isolate pure populations of leukocyte subsets from whole blood has attracted a lot of interest for point-of-care diagnositics. While the principle behind a cell isolation approach can be easily adapted to a wide spectrum of clinical applications, detecting these isolated cells remains a technical challenge to be addressed.
The use of optical microscopy for detection and quantification of surface immobilized cells within microdevivces does not represent the optimal solution for point-of-care applications. This is because optical detection methods depend on a stable light path, lensing, filtering, and focusing mechanisms that could add cost and complexity to detection. In addition, optical detection tends to be low throughput, because of the small detection area available at a single time. At the same time, the most commonly used cell counting strategies like flow cytometry and impedance measurement (i.e., Coulter counters) cannot be applied to cells attached on surfaces, despite miniaturized platforms having been implemented by several researchers. Alternative techniques to detect attached cells by substrate impedance sensing require cell coverage on the electrode surface to reach near unity for detectable measurements. Studies using non-optical methods to detect few cells on large surface areas in a relatively large volume—including even the microliter volumes of microscale devices have not yet been reported, despite the need for non-optical detection methods in microfluidic applications.
Detection and enumeration of cells are essential for medical diagnostics, especially AIDS, cancer diagnosis, and pathogen detection. While most existing methods to detect cells are optical (i.e., microscopy), electrical detection is significantly simpler, cheaper, and more amenable to point of care devices. To date, electrical detection and enumeration of intact cells based on impedance spectroscopy (i.e., detection of changes in electrical impedance caused by the presence of cells) have proven to be extremely practical and inexpensive, but limited to large cell populations or homogenous cell types (e.g., Coulter counting of red blood cells).